Unless your humidity trays are in a very small enclosure with your plants, they are not going to provide much or any increase in ambient humidity. Sorry. They are a much touted "solution" that in reality does almost nothing in a standard home environment with standard air circulation.

Humidity or pebble trays DO increase ambient humidity!! Even staging a few glasses or jars of water in amongst your houseplants will increase humidity. In fact, just grouping your houseplants close together will increase humidity (all plants transpire and release water vapor). So will misting but not enough to make a measurable impact unless you do so very often. Any water source nearby - but something the plants are not sitting in directly - will help to elevate humidity. The closer to the plants, the greater the benefit/higher the humidity level.

Hydrospikes are an inefficient watering device or soil moisture monitor. They have absolutely NO impact on ambient humidity.

For most practical conditions humidity trays do not do much to raise humidity. Al has written extensively about his own measurements. I have measured many times under various conditions and concur fully with Al on this subject. The only place where humidity will be higher is about an inch or two above the tray with no air flowing. In a sealed room, it is possible, otherwise not. Physics does not support it.

Whatever water evaporates from the humidity tray will quickly dissipate into the surrounding. One cannot simply have higher humidity in a small zone for too long where the rest the environment is dry. To claim that you can maintain a bubble of high humidity near a tray with water while the rest of the environment is dry is just not possible. The very fact that a bubble of denser air exists will cause a air flow and whisk it away.

The only practical way of increasing humidity is in an enclosed environment where water in the air cannot escape easily. And use a humidifier - A humidifier actively pumps water into the air which is not possible with tray of water. There is not enough energy in the plain water to sufficiently and rapidly inject water into air.

Average humidity of Florida is 70% or so even though it is surrounded with water. Alaska has the highest average humidity in US.

Instead of looking for a solution to increase humidity only around your ferns, why not look for ways to raise the humidity in the entire room - 40-60% humidity promotes good health not only in most plants, but also in humans.

Below 40% humidity stimulates proliferation of dust and respiratory viruses, and above 65-70% of mold spores, fungi, and mildew. So yeah, even though I love my plants, I wouldn’t keep any that require humidity levels outside of the ideal range for me and my family; allergies, asthma, and resp. infections aren’t really something I’m willing to battle for the sake of growing plants indoors.

So then, if you have dry air, how can you raise humidity in a room in order to reach that sweet spot of 40-60%? The only way I know how is by using humidifiers; preferably the steam ones, since cool-mist humidifiers can potentially disperse germs and allergens in the aerosol. A pot of boiling water would also work.

However, before you do any of the above, you need to get a reliable hygrometer.